Gzowski was born in Saint Petersburg to a noble Polish father who served with the Russian military. In 1830, he emigrated with his family to the United States after the Polish revolt against Russia. He knew no English, but began to study law and was admitted to practice. His father was an engineer, and as this became his primary interest, Kazimierz became involved in railway construction in the United States. Eventually, he was hired as an engineer to help in the construction of the New York and Erie Railway.

In 1841 he moved to Canada to work on the Welland Canal, and also helped finish the building of Yonge Street and other projects, for the Department of Public Works in southern Ontario. He settled in London. He then formed his own company that built the Grand Trunk Railway from Toronto to Sarnia (1853-57) and the International Bridge across the Niagara River at Fort Erie.

Keenly concerned over the defence of Canada, Gzowski fought for the strengthening of the Canadian militia and served in it as lieutenant-colonel and colonel. He took an active interest in military affairs and in 1872 was appointed "Staff Officer of the Militia of Canada" as a lieutenant colonel and Staff Officer to the Engineer Force of the Dominion two years later. In 1879, he was promoted to colonel and appointed Honorary Aide-de-Camp to Queen Victoria. He was knighted in 1890.

Among his other accomplishments, Colonel Gzowski was the first chairman of the Niagara Parks Commission (1885-93), was a founder of the Canadian Society of Civil Engineers in 1887, and served as Administrator of Ontario (1896-97).

Sir Casimir Gzowski is remembered in a park in Toronto bearing his name and by a plaque at the International Bridge across the Niagara River at Fort Erie.