Siaka Probyn Stevens, leader, secretary-general, and one of the founders of the APC, was born on August 24, 1905, in Moyamba. He was educated at the Albert Academy in Freetown, and on leaving school, in 1923 he joined the police, where he rose to the rank of a First Class Sergeant and Musketry Instructor by 1930. From 1931 to 1946, he worked on the construction of the DELCO railway linking the Port of Pepel with the iron ore mines at Marampa, where he also served in various capacities, including station manager and stenographer. In 1943 he helped co-found the United Mine Workers Union and served for fifteen years as the Union's first full-time secretary-general. Subsequently, he was appointed to the Protectorate Assembly to represent workers’ interests. And in 1947, Stevens won a British Council scholarship to study industrial relations at Ruskin College in Oxford for nine months, and he spent the next six months attending union meetings, working in union branch offices, and visiting mines under the auspices of the British Trades Union Congress in London. Beyond that, he was secretary-general of the Sierra Leone Trades Union Congress from 1948 to 1950. Stevens also served on the Moyamba District Council for seven years.

Stevens was a founding member of the SOS that merged with the PP to form the SLPP in 1951 under the leadership of Sir Milton. In addition, Stevens was elected to the Legislative Council by the Protectorate Assembly in 1951. And he was appointed the country's first minister of lands, mines, and labor in 1952. Moreover, he was the chief negotiator in the SLST–Sierra Leone diamond mining talks of 1956, which brought the country bad publicity and cost taxpayers Le3 million. As minister he oversaw the successful introduction of the program that opened diamond mining to small-scale local miners. But his tenure also witnessed the rapid growth of illicit digging and smuggling and faced the worst strike in the country's history. According to Stevens, “I sometimes think that the only real division in this country is between those who do and those who do not make money out of illicit diamonds.” Stevens's political career suffered a major setback in 1957 when he lost his parliamentary seat after an election petition.