In 1978, while studying print-making in Hamburg, Gavin Jantjes was deemed a persona non grata by South Africa. His crime? Creating a series of prints that documented the violence and discrimination in the apartheid state, which he gathered together under the tongue-in-cheek name of a "colouring book".

“South Africa wanted me extradited,” he tells The National at the opening of his major exhibition in London. “I never went home again for the next 25 years.”

Jantjes, whose exhibition To Be Free! A Retrospective of Gavin Jantjes is on show at the Whitechapel Gallery, was already in the anti-apartheid movement and was granted asylum by Germany in 1973. But his art – A South African Colouring Book – got him banned.

The terrific show at the Whitechapel tells the story of Jantjes’s art and activism over the course of five decades. And it also raises crucial questions about the role of politics in art – and what kind of burden that places on the artist.

“The title of the exhibition – To Be Free – is really important,” says the Whitechapel’s director Gilane Tawadros, who has known Jantjes for years. “Gavin occupies a singular position in British and international art history, as an artist and as an activist and curator. What’s important is that he has been so attuned to what an artist needs to be at different moments in history.”

Jantjes made his early screen prints in the 1970s as crackdowns in South Africa were becoming increasingly brutal. The works bristle with the urgency of testimony in their need to make the public aware of the realities of apartheid.

Headlines from newspapers share space with images of violence. In Freedom Hunters (1977), a young man is repeated twice – in an echo of Andy Warhol’s Elvis screen prints – holding what appears to be a rubbish bin lid as a shield. Barbed wire bisects the work, while media images of young protestors show the scale of resistance.

But from this moment onwards, the show reveals Jantjes's artworks move further away from politics – and ends in pure abstraction. It is like watching a character shed coat after coat, until finally floating away.

After working in screen printing, Jantjes turned to oil painting. In part because, he says, his wife didn’t want him bringing his new baby to a studio full of heavy machinery. The composition of these early paintings is still collage-like, carrying multiple items in the picture frame and maintaining the role of witness. Amaxesha Wesikolo ne Sintsuku (Schooldays and Nights, 1978) documents the Soweto uprising, when police opened fire on schoolchildren, killing hundreds. In Jantjes’s painting, a white man looks askance at three black figures: a woman falls into the centre of the work, a father who seems in shock or about to shout and a child lying lifeless on the ground.

Then Jantjes changed tack once again. In the 1990s, he created three major bodies of work in oblique, imagined settings on slavery, African tradition and the extractive relationship between the West and Africa.

“I wanted to get away from constantly talking about apartheid,” he recalls. “Firstly, I chose a different subject. This was the history of slavery, in the seven pictures of the Korabra series. These were more of what I call poetic pictures. A lot of it was invented, instead of being taken from photographs.”

He also began playing around with material – an artistic shift for which he again credits his safety-conscious wife. She wanted him to stop painting in oil, because she worried about turpentine being carcinogenic (turpentine is used to remove oil paint). He agreed and, at an anti-apartheid exhibition in Paris, he happened to ask the noted Spanish painter Antoni Tapies where he bought his acrylic paints. Tapies explained he made it himself, by simply mixing pigment with glue.

Jantjes realised he could achieve extremely saturated, bright paint from this technique – then refined it further. To depict the landscape of slavery, he added materials such as sugar and cotton to the paint – making the cash crops, for which the plantation owners of the American South bought enslaved Africans, part of the work itself.

For the Zulu series, he looked at the night sky as the single shared entity between all Africans, offering it as a measure of optimism rather than censure. And alongside some of his paintings, he made small wax statues like votive offerings.

“The question was how to make work but have it not be determined by politics,” he says. “How do you represent colonialism without making an obvious statement, like in the images of the uprising? You look at the hobbies of the colonialists and you reduce colonialism to that classic top hat and umbrella – or you make a link of slave chains, but on fire.”

Jantjes as curator and advocate

To Be Free! opened last year at the Sharjah Art Foundation, curated by Salah Hassan, the director of the Africa Institute. It appears at the Whitechapel in collaboration with Tawadros and Cameron Foote and is in some sense a homecoming. Jantjes played a leading role among the cultural figures in London of the 1980s who opened up space for artists from Africa, Asia, the Caribbean and the Middle East – who had, up to that point, been largely ignored by the British art world.

He co-curated the exhibition From Two Worlds, in which many diaspora artists showed and which appeared at the Whitechapel in London and was instrumental in setting up the Institute of International Visual Arts, or Iniva, for which Tawadros served as founding director.

He also participated in The Other Story, the celebrated exhibition at the Hayward in 1989, which was put together by the artist Rashed Araeen. Among his six works was his now-famous untitled painting from 1989 that shows a Fang mask exhaling a breath that reaches behind it to encircle a female figure from Pablo Picasso’s Les Demoiselles d’Avignon (1907), in a clear comment on western modernism’s African debts. Later, as a curator and director at different institutions, he gave major shows to artists such as Ghada Amer and Susan Hiller.

Ultimately, however, championing other artists took away from his own output.

“He felt that he couldn't be a curator or artistic director and be a painter at the same time,” says Tawadros. "So he decided to stop painting while he was working as artistic director [from 1998 to 2004] at Henie Onstad [outside of Oslo].”

Finally, after a long hiatus, in the mid-2010s, he returned to painting. The works are a surprise, even if the narrative – of a move towards abstraction – has been the leitmotif of the exhibition. They are ethereal swirls of paint in pastel, even hallucinogenic colours, that waft onto the canvas.

“They lose all subjectivity,” says Jantjes of the acrylics, which he builds up in thin layers."

Tawadros says: "These really dreamy, poetic works have no obvious subject matter. They're just inviting you to have a conversation with the artwork – not to have a political or historical narrative, but simply to be with the work. For me, the exhibition is a journey about what political and artistic freedom means.”