On July 4, 1873, citizens concerned for both the welfare of animals and children founded the Ontario Society for the Prevention of Cruelty to Animals (Ontario SPCA). Several years later, the Children's Aid Society was established as a separate organization and the Ontario SPCA's primary focus became bringing animal cruelty cases to the attention of authorities.

In 1887, the Ontario SPCA's lobbying efforts resulted in a decision by the Ontario Board of Police Commissioners to appoint a police officer to deal with animal cruelty issues on a full-time basis. In 1919, the first legislation was passed to protect animals. The Ontario SPCA's Inspectors and Agents were given authority by the government, through the Ontario SPCA Act, to investigate situations involving animal abuse.

In 1955, the Government of Ontario repealed the Act of 1919 and replaced it with new legislation. Inspectors and Agents were granted the power to enter property, to carry out investigations and given broader powers to remove animals that were believed to be in distress or in need of care and attention. In order to obtain enforcement powers, many independent societies affiliated with the Ontario SPCA, enabling the Society to become a larger, more linked group that could affect better humane laws and conditions.

In 2008 the first comprehensive amendment to the Ontario SPCA Act since its inception passed, transforming Ontario's weak animal protection laws into some of the toughest animal protection laws in the country.

Today the Society is comprised of more than 50 Ontario SPCA Communities providing animal protection, rehabilitation and care; advocacy and humane education in neighbourhoods across the province.