A quick Google resulted in this information on mercenaries and the Geneva Convention:

Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts (Protocol I), 8 June 1977.

Part III : Methods and means of warfare -- Combatant and prisoner-of-war status #Section II -- Combatant and prisoner-of-war status



[p.571] Article 47 -- Mercenaries


[p.572] General remarks

1789 The problem of mercenaries was first raised at the United Nations in1961 in connection with the Katangese secession. (1) Later on, in 1964, the Congolese government itself recruited mercenaries to suppress an insurrection. When they were subsequently instructed to lay down their arms, most of them refused to do so and openly rebelled against the government (1967). The latter then called upon the Security Council, as well as the Organization of African Unity (OAU), to which it had already appealed in 1964. The Security Council (2) and the Conference of Heads of State and Government of the OAU requested States to prevent the recruitment of mercenaries in their territory for the purpose of overthrowing the governments of foreign States. The epilogue to this unhappy affair took place in Rwanda, where the mercenaries eventually sought refuge. They were repatriated with the help of the ICRC, on condition that they undertook not to return to the African continent. (3)

1790 Since then, there has scarcely been any conflict involving military operations in which the presence of mercenaries has notplayed a part in one way or another. Nevertheless, since 1968 the United Nations General Assembly has adopted a firm position stating that the practice of employing mercenaries against national liberation movements is a criminal act, (4) and the mercenaries themselves are criminals. In 1977 it was once more the Security Council which adopted, by consensus, a resolution condemning the recruitment of mercenaries with the objective to overthrow governments of Member States of the United Nations. (5) Also in 1977 the Council of Ministers of the OAU adopted a Convention for the Elimination of Mercenarism in Africa at its 29th session in Libreville. (6) Based partly, as regards the definition of the term "mercenarism" as such, on previous [p.573] drafts, (7) and, with the exception of the problem of payment, on the definition of the term "mercenary" given in the present Article 47 , this Convention was a response to the concern of those who see the text of the Protocol as paving "the way for the conclusion of more stringent regional instruments", (8) on the assumption that Article 47 was only "the first, and that other more satisfactory international texts would follow". (9) In fact, this OAU Convention of 1977 was an attempt to respond to the wishes of some delegations who had participated in the Diplomatic Conference, wishes which could not be met by the demands of the inevitable compromise. It condemns the mercenarism as such, and not only the mercenary himself (Article 1 , paragraph 2). It contains a pure and simple prohibition on according a mercenary the status of combatant and prisoner of war (Article 3 ). Finally, the definition of the term "mercenary" diverges from that of the Protocol on one point, as stated above. (10) At the time of writing, a draft of an "international Convention against the recruitment, use, financing and training of mercenaries" is being formulated within the United Nations. (11)