In 1834 he was elected to the 12th Parliament of Upper Canada, representing the third riding of York County as part of the reform movement.

Morrison had been an early supporter of the reform movement in Upper Canada and participated in meetings to encourage political change.

He spoke repeatedly against armed rebellion and encouraged William Lyon Mackenzie to continue soliciting support for his reforms.

When amnesty was granted to most participants of the rebellion in 1843, he returned to Toronto to continue his medical practice and serve on various boards of medicine.

[1] However, he might have been dismissed because he converted to Methodism in 1818 and Upper Canadian government officials thought he deviated from proper Anglican values.

He ran on a reform platform supported by William Lyon Mackenzie, which opposed the firing of Judge John Walpole Willis by Lieutenant Governor Peregrine Maitland from the Court of King's Bench.

[6] After the election, Morrison helped organise a committee to petition British politicians to intervene in Upper Canada to protect the constitution.

[7] On August 22, 1832, he opened the York Dispensary with William Warren Baldwin and John E. Tims, but it closed a year later because of a lack of funding.

Morrison organised a meeting on January 19 to demand the lieutenant-governor call an election to enable Hagerman and others to campaign and explain their actions to voters in Upper Canada.

[9] Mackenzie was in England during the subsequent by-election for his Legislative Assembly seat, so Morrison spoke on his behalf at the poll and encouraged others to join his recently created Upper Canada Central Political Union.

[15] Morrison quarrelled with Tory members over accusations that Mackenzie, the city's mayor, was interfering with the work of health officers.

Tory newspapers speculated of an argument between Mackenzie and Morrison over the lieutenant governor's offer to fund the Cholera Hospital.

His mayoralty focused on the conflict between Reformers and Tories in the provincial legislature, but his term was also credited with new infrastructure projects to improve the city's water supply and an increased number of gas-powered street lamps.

[8] His council also approved funding to pave major roads with macadam and created a House of Industry, which allowed for the creation of workhouses the following year.

When Mackenzie suggested a revolt in October 1837, Morrison reportedly exclaimed, "This is treason; if you think to entrap me into any such mad scheme, you will find I am not your man.

[23] A loyalist militia arrested Morrison on December 6 for participating in the Upper Canada Rebellion and held without bail until his trial in April 1838.

The defence counsel also challenged the idea that the declaration could call for the death of the monarch if the king was not physically present in the city.

[29] Half an hour into their deliberations, the jury inquired about finding Morrison guilty of a crime less serious than high treason, but they were informed that they could not.