He continued in British films during the 1960s, portraying stereotypical roles, such as a butler in the film Woman of Straw (1964), and in other films, such as East of Sudan (1964), Khartoum (1966) and The Last Safari (1967). He also appeared on television, in programmes such as The Human Jungle (The Twenty-Four Hour Man, Series 2 Episode 3) first aired 26.10.64 playing Alvar Bell, Z-Cars, Dixon of Dock Green, Gideon's Way, Danger Man, and a 1968 episode of The Avengers. In 1968, he also played the lead role in a West End production of Night of Fame. According to his obituary in The Times, this was the first time that a black actor had played a role written for a white man in English theatre. He was seen as a British equivalent to Sidney Poitier, and was frustrated that actors who started out at around the same time as him – such as Sean Connery, Terence Stamp, Michael Caine, Tom Courtenay and John Hurt – had become stars, and he had not.

He arrived in London, England in 1952, and served for two years in the Royal Air Force, where he first received the nickname "Johnny", but then Caribbean actor Earl Cameron persuaded him to become an actor, and he attended RADA. He became a stagehand at the Royal Court Theatre, and appeared on stage in various plays from 1958. He had a small part in the 1958 film version of Look Back in Anger, directed by Tony Richardson, who had seen him on stage. He took a leading role in the 1961 film Flame in the Streets, playing the Jamaican boyfriend of the (white) daughter (played by Sylvia Syms) of a liberal working-class trades unionist (played by John Mills). He lived for a period in Paris, where he met his wife, Cecilia Enger.

Sekka eventually moved to the United States with the aim of getting better roles. He had a minor part in the films A Warm December (1972) and Uptown Saturday Night (1974), both directed by Poitier. The first also featured Earl Cameron and the second Bill Cosby and Richard Pryor. These roles led to a more memorable role in the sitcom Good Times, where he portrayed Ibe, Thelma's (BernNadette Stanis) African love interest. In 1976, he starred in the movie Mohammad, Messenger of God (also known as The Message) about the origin of Islam and the message of Muhammad, in which he played Muhammad's Ethiopian disciple Bilal al-Habashi. He appeared in the 1982 film Hanky Panky, and played Banda in the 1984 miniseries Master of the Game.

He was not cast in Roots (1977), being considered insufficiently American, but secured a role in the sequel, Roots: The Next Generations (1979), playing an African interpreter. Sekka is widely known among science fiction fans for his role as Dr. Benjamin Kyle in the television series Babylon 5's pilot movie, The Gathering (1993). Recurring health problems forced him to decline a Future role in the series, and ultimately were the reason he retired from acting altogether.