Maine Coons are the largest breed of domestic cat. Males weigh from 7 to 10 kg with females weighing from 4 to 6 kg.The height of adults can vary between 25 and 41 cm and they can reach a length of up to 100 cm, including the tail, which can reach a length of 36 cm and is long, tapering, and heavily furred, almost resembling a raccoon's tail. The body is solid and muscular, which is necessary for supporting their own weight, and the chest is broad. Maine Coons possess a rectangular body shape and are slow to physically mature; their full potential size is normally not reached until they are three to five years old, while other cats take about only one year.

In 2010, the Guinness World Records accepted a male purebred Maine Coon named "Stewie" as the "Longest Cat" measuring 123 cm from the tip of his nose to the tip of his tail.

The Maine Coon is a longhaired, or medium-haired, cat. The coat is soft and silky, although texture may vary with coat color. The length is shorter on the head and shoulders, and longer on the stomach and flanks with some cats having a lion-like ruff around their neck. Minimal grooming is required for the breed, compared to other long-haired breeds, as their coat is mostly self-maintaining due to a light-density undercoat. The coat is subject to seasonal variation, with the fur being thicker in the winter and thinner during the summer. Maine Coons, due to their large size, have larger claws. There have been cases of Maine Coons using their claws to grip into walls.

Maine Coons can have any colors that other cats have. Colors indicating hybridization, such as chocolate, lavender, the Siamese pointed patterns or the "ticked" patterns, are unaccepted by breed standards.The most common color seen in the breed is brown tabby. All eye colors are accepted under breed standards, with the exception of the occurrence of blue-colored or odd-eyes (i.e., two eyes of different colors) in cats possessing coat colors other than white.

Many of the original Maine Coon cats that inhabited the New England area possessed a trait known as polydactylism (having one or more extra toes on the feet).While some sources claim that trait is thought to have occurred in approximately 40% of the Maine Coon population in Maine at one time, little evidence has been given to substantiate this claim.Polydactylism is rarely, if ever, seen in Maine Coons in the show ring since it is unacceptable by competition standards.The gene for polydactylism is a simple autosomal dominant gene,which has shown to pose no threat to the cat's health.The trait was almost eradicated from the breed due to the fact that it was an automatic disqualifier in show rings. Private organizations and breeders were created in order to keep polydactylism in Maine Coons from disappearing.

Maine Coons are known as the "gentle giants"and possess above-average intelligence, making them relatively easy to train.They are known for being loyal to their family and cautious—but not mean—around strangers, but are independent and not clingy.The Maine Coon is generally not known for being a "lap cat" but their gentle disposition makes the breed relaxed around dogs, other cats, and children. They are playful throughout their lives, with males tending to be more clownish and females generally possessing more dignity, yet both are equally affectionate.Many Maine Coons have a fascination with water and some theorize that this personality trait comes from their ancestors, who were aboard ships for much of their lives.Maine Coons are also well known for yowling, chattering, chirping, "talking" (especially "talking back" to their owners), and making other loud vocalizations.