Yes. Mercenaries were used, and were extremely common in a wide variety of post-colonial wars, especially in Africa. The Biafra War was one of those where mercenaries, mostly British, French, or White Africans (that is South Africans, Rhodesians, Portuguese Angolans or Mozambicans, etc), but also Scandinavians, Eastern Europeans, Americans, and many others, were central to this conflict.

The reasons for the presence of mercenaries was manyfold. In many of these newly independent states, there was no military or administrative tradition, and as such western advisers and/or mercenaries had to be brought in to, at least initially, help with the transition of government, to create the military, and things of that nature. Further, secessionist movements, such as happened in Biafra in Nigeria, or in Kasai, and Katanga in Congo-Leopoldville, were often backed by Western corporations, who had no militaries of their own, but had money. And this money was used to buy soldiers that were willing to fight in Africa. At this time, the mercenary trade was a common profession for a wide range of former soldiers, veterans largely of the Second World War, or Korea, or any of several colonial conflicts such as the Malay Emergency, or the war in Algeria. Many (but far, far, from all, I should stress, this is a trend, not a rule) were veterans who preferred a life of martial adventure over a droll civilian existence. Others still were Eastern Europeans. Czech, Polish, Balkan, expats who had fought in the British-supplied armies in exile during the Second World War, who had martial experience, but no home that they wanted to return to as their homelands were now under the Communist yoke, and many of them saw little functional difference between the fascist occupation and the Soviet occupation of their homelands.

Many of these mercenaries were hired by British, American, and French intelligence agencies, and indeed the first "modern" private military companies (KMS, WatchGuard International) grew out of the British operations in Yemen and Oman in this same period.

In terms of the Biafra Conflict, there were mercenaries on both sides, but especially amongst the Biafrans, who were supported by the French. Many of these mercenaries had fought in numerous other conflict zones. Many of them had recently come from the Katangan secession in the Congo, or had fought in the Middle East, in Oman and Yemen. Many of them would later go on to fight in Angola, where after the expulsion of the Portuguese, many would fight for UNITA in the civil war that followed the country's independence.

It has been quite a time, so I'll have to go dig it up, but I've written rather notably in the past here on the role of mercenaries in African conflicts during this period. Once I find the posts, I will put up a reply with links to those past answers.

Just for the record, OP, in the song Roland is referenced as having "fought the Congo War" from '66 to '67 - this likely refers to the Kisangani Mutinies, technically considered a separate conflict from the Congo Crisis proper, generally demarcated as from 1960 to 1965. Biafra, in this case, is likely referenced as Roland's port of entry onto the continent. It's unclear if Roland, whether alive or as a revenant, participated in the Biafran War, although that wouldn't be a bad assumption to make.

Some of the answer might be found in what /u/aquatermain had to say about Why did the Nigerian Civil War (the Biafran War) have such a bizzare amount and combination of countries as belligerents for each side?

Just to piggyback off this question, how accurate was Frederick Forsyth’s portrayal of the situation for mercenaries in African wars of the era in The Dogs of War?