St Bernard Mastiff Mix: Size, Temperament, and Care Guide

What Is a St Bernard Mastiff Mix?

The St Bernard Mastiff mix — sometimes called a "Saint Mastiff" — is a cross between two of the largest dog breeds in the world: the St. Bernard and the English Mastiff (or occasionally the Bullmastiff, which changes the mix's profile significantly). This is not an AKC-recognized breed. It falls into the designer or mixed-breed category, meaning there is no breed standard, no registry, and no guarantee of what any individual puppy will look like or act like.

If you're researching this mix, you're likely drawn to the idea of a giant, gentle companion. And you may get one. But with two parent breeds that each carry serious health risks and strong behavioral traits, you need to go in with clear eyes. This guide breaks down what you can reasonably expect — and what you absolutely cannot predict.

Parent Breed Breakdown: St. Bernard vs. English Mastiff

Understanding the parent breeds is the single most useful thing you can do before committing to this mix. Every trait your dog displays will trace back to one of these two lineages.

The AKC St. Bernard breed standard describes a dog bred for endurance in extreme cold, with a strong retrieving instinct and a famously gentle disposition toward children. The AKC English Mastiff standard describes a massive guardian breed that is dignified rather than aggressive, but deeply bonded to its family and reserved with strangers.

When these two breeds are combined, you're blending a mountain rescue dog with an estate guardian. The result is almost always a large, loyal, low-energy companion — but the specifics vary enormously.

How Big Will a St Bernard Mastiff Mix Get?

This is one of the most common questions, and the honest answer is: very large, but the exact size is unpredictable. Both parent breeds are giants, so you're guaranteed a big dog. But the range is wide.

A male that inherits more Mastiff genetics, particularly from a large sire line, can exceed 200 lb (91 kg) by 18 months. A female leaning toward the St. Bernard side may plateau around 140 lb (64 kg). Growth plates in giant breeds don't fully close until 18–24 months, so expect slow, steady growth well past the first year.

Generations Explained: F1, F2, and Backcrosses

If a breeder uses generation terminology, here's what it means for this mix:

For most buyers, you'll encounter F1 puppies. This is generally preferable because you at least know the parentage. Be wary of breeders who cannot identify the specific parent breeds or who claim multigen status without documentation — this often masks unknown ancestry.

Temperament: What to Expect (and What You Can't Predict)

Both parent breeds are known for being gentle giants, which is encouraging. But "gentle giant" doesn't mean "easy dog." Here's a realistic range:

Best-Case Scenario

Your mix inherits the St. Bernard's eagerness to please and the Mastiff's calm steadiness. The result is a dog that is deeply bonded to family, patient with children, relatively easy to train with positive reinforcement methods, and content to lounge for most of the day. This dog greets visitors with a wagging tail or polite indifference and settles quietly in the home.

Challenging Scenario

Your mix inherits the Mastiff's guardian instincts and the St. Bernard's stubbornness. You get a 170 lb dog that is suspicious of strangers, resistant to training, prone to leaning against doors to block guests, and too large to physically manage if it decides not to move. Mastiff-line dogs can develop territorial behavior if not socialized extensively during the critical 3–14 week window.

Temperament Variability Chart

One trait that is nearly universal in this mix: low exercise drive combined with high space requirements. These dogs don't need to run, but they do need room to move comfortably. A 180 lb dog trying to navigate tight hallways or steep stairs is a recipe for joint injury — for the dog and for you if you try to assist.

Coat Types, Shedding, and Grooming

Is the St Bernard Mastiff mix hypoallergenic or low-shedding? No. Neither parent breed is hypoallergenic, and this mix will shed — the only question is how much.

Here's what's possible depending on which coat genetics your dog inherits:

• Short Mastiff-type coat: Dense, close-lying hair that sheds moderately year-round. Easier to maintain — weekly brushing with a rubber curry brush (like a Kong ZoomGroom, ~$8) keeps loose hair managed. This is the most common outcome since both parents can carry short-coat genetics.
• Short St. Bernard-type coat: Slightly longer than Mastiff coat, with a denser undercoat. Seasonal shedding blowouts in spring and fall are intense. You'll need an undercoat rake (e.g., Furminator deShedding Tool for giant breeds, ~$35–$45) during peak shedding.
• Long-haired St. Bernard coat: If the St. Bernard parent is the long-haired variety, puppies may inherit medium-to-long wavy fur. This requires brushing 3–4 times per week to prevent matting, especially behind the ears, under the legs, and around the hindquarters. Professional grooming every 8–10 weeks may be necessary ($60–$90 per session for a giant breed).

Drool management is non-negotiable. Both parent breeds are among the heaviest droolers in the canine world. Expect to keep drool rags in every room. Leather furniture will show water spots. Walls near where your dog shakes its head will need regular wiping. Some owners use drool bibs or bandanas during high-drool periods (after eating, drinking, or exercise).

Basic Grooming Schedule

Health Considerations: Hybrid Vigor Myth vs. Reality

Let's address the most persistent myth in the designer dog world: "hybrid vigor" does not eliminate health problems. The concept of heterosis — where crossbred animals show improved biological function — has some basis in science, but it primarily applies when crossing genetically distant populations to reduce the expression of recessive disorders. It does not protect against conditions that both parent breeds share.

Unfortunately, the St. Bernard and the English Mastiff share several serious health risks. Your mix is susceptible to all of them.

Inherited Health Conditions to Screen For

Any responsible breeder of this mix should provide OFA (or equivalent) health clearances for both parent dogs, covering hips, elbows, cardiac, and eyes. Ask for documentation — not verbal assurances. The Orthopedic Foundation for Animals database allows you to verify clearances by dog name or registration number.

Always consult your veterinarian to develop a health screening plan tailored to your individual dog, including when to begin cardiac monitoring and whether prophylactic gastropexy is appropriate.

Who Is This Mix Best Suited For?

This is not a beginner's dog. It's also not a good fit for everyone who has owned dogs before. Here's an honest assessment:

Good Fit If You:

• Have experience with giant or strong-willed breeds
• Live in a home with ground-floor access (minimal stairs) and a fenced yard
• Can afford $3,000–$6,000+ annually in food, preventive care, and potential medical costs
• Are home most of the day or can arrange mid-day companionship
• Have the physical ability to assist a 150+ lb dog if it becomes injured or unable to stand
• Understand and accept a lifespan of 6–10 years
• Don't mind drool, shedding, and a dog that takes up an entire couch

Poor Fit If You:

• Live in an apartment or rental with breed/size restrictions
• Work long hours away from home regularly
• Have very young children (a 170 lb dog can accidentally injure a toddler simply by leaning or walking past)
• Are a first-time dog owner
• Cannot physically manage a dog that may outweigh you
• Want a hiking or running partner — this mix is not built for sustained high-intensity exercise

Exercise Requirements

Despite their size, most St Bernard Mastiff mixes are relatively low-energy. Plan for:

• 30–45 minutes of moderate activity daily (leisurely walks, gentle yard time)
• Avoid high-impact exercise (running on pavement, jumping, agility) to protect developing joints in dogs under 24 months and aging joints in seniors
• Swimming is excellent low-impact exercise if your dog enjoys water — but many Mastiff-line dogs are not natural swimmers due to their heavy front-end build. Always supervise and use a canine life jacket (e.g., Ruffwear Float Coat, ~$90) for safety
• Avoid exercise in heat above 80°F (27°C) — both parent breeds are extremely heat-sensitive due to their brachycephalic tendencies and thick coats. Heatstroke is a real risk

Cost and Availability: Breeders vs. Rescue

The St Bernard Mastiff mix is not a common designer breed. You won't find dedicated breeding programs the way you would for Labradoodles or Goldendoodles. This affects both availability and pricing.

Puppy Pricing

Because this is an uncommon mix, your best strategy is to:

1. Search breed-specific rescues: St. Bernard Rescue Foundation and Mastiff Club of America Rescue sometimes take in mixes
2. Monitor Petfinder and Adopt-a-Pet using both parent breed names as search terms
3. Network with local giant-breed rescue groups — they often know about available dogs before they're listed publicly
4. If pursuing a breeder, verify they health-test both parents and are willing to share OFA certificates

First-Year Ownership Cost Estimate

Ongoing annual costs after the first year typically run $2,500–$4,500 for a healthy dog. If your dog develops hip dysplasia, cardiac disease, or requires bloat surgery, costs can spike dramatically. Pet insurance for giant breeds runs $70–$130/month depending on coverage level and deductible — consider enrolling early before any conditions are documented as pre-existing.

Frequently Asked Questions

What two breeds make up a St Bernard Mastiff mix?

A St. Bernard and an English Mastiff (or sometimes a Bullmastiff). Always confirm with the breeder or rescue which specific Mastiff-type breed was used, as Bullmastiffs are smaller (100–130 lb) and will produce a smaller mix than English Mastiffs.

How big will my St Bernard Mastiff mix get?

Most adults fall between 140–200 lb (64–91 kg) and 27–30 inches (69–76 cm) at the shoulder, but individuals outside this range are possible. Males are typically larger. Expect full size by 18–24 months.

Is this mix good with children?

Generally yes — both parent breeds are known for patience with children. However, the sheer size creates an accidental injury risk. A 170 lb dog turning around in a doorway can knock over a small child. Always supervise interactions and teach children not to climb on or startle the dog.

Does this mix shed a lot?

Yes. Shedding ranges from moderate year-round (Mastiff-type coat) to heavy seasonal blowouts (St. Bernard-type coat). Neither parent breed is hypoallergenic. If anyone in your household has dog allergies, this mix is not a good choice.

How long do St Bernard Mastiff mixes live?

Expect 6–10 years, with most living 7–9 years. Giant breeds age faster than smaller dogs. Cancer (particularly osteosarcoma) and cardiac disease are the leading causes of death in both parent breeds. Regular veterinary checkups, maintaining a lean body condition score, and early detection of health issues can help maximize your dog's lifespan.

Can I keep this mix in an apartment?

Technically possible but not recommended. A dog this size needs space to move comfortably, and many apartment complexes have weight restrictions (often 50–75 lb limits). Stairs are also a concern — carrying or assisting an injured 180 lb dog down multiple flights is dangerous for both of you. A single-story home with a fenced yard is ideal.

What should I feed a St Bernard Mastiff mix?

Choose an AAFCO-approved giant-breed formula. For puppies, a giant-breed puppy formula is essential — these are formulated with controlled calcium and calorie levels to prevent rapid growth that damages developing joints. Brands like Purina Pro Plan Giant Breed, Royal Canin Giant, and Hill's Science Diet Large Breed are widely available. Feed measured portions to maintain a body condition score of 4–5 out of 9 (you should feel ribs without pressing hard but not see them prominently).