The state-level rules every owner and walker in Massachusetts should know. Local leash lengths, licensing and off-leash rules are set by each city — find those on the city pages below.
Massachusetts imposes strict liability on the "owner or keeper" of a dog, covers non-bite injuries, and is extra-protective of young children — so a walker holding the leash is a strictly-liable keeper.
Massachusetts (M.G.L. c. 140, § 155) is a strong strict-liability state, and it names the keeper right alongside the owner: the owner or keeper is liable for any damage a dog does to a person or property, regardless of the dog's history or the owner's care. The only defenses are that the victim was trespassing, or teasing, tormenting, or abusing the dog. Courts define a keeper as someone harboring with an assumption of custody, management, and control of the dog (Maillet) — a dog walker with the leash fits.
The statute covers non-bite injuries — knockdowns, a dog breaking its leash and causing a fall, even property damage. Children under 7 are presumed not to have trespassed or provoked, flipping the burden to the defendant — very protective of kids. And a dog previously declared dangerous that injures again exposes its owner to treble (3×) damages (§ 159).
Leash rules are largely local — state law only mandates leashing in highway rest areas (§ 174B) — and a local violation is negligence per se. The personal-injury limit is three years.
This is general information about Massachusetts law, not legal advice. Confirm current rules with the official state and municipal sources.