[4][5][6] Machar obtained a PhD in strategic planning in 1984 and then joined the rebel Sudan People's Liberation Movement/Army (SPLM/A) during the Second Sudanese Civil War (1983–2005).

In 2000 he left the SSDF and formed a new militia, the Sudan People's Defense Forces/Democratic Front (SPDF), and in 2002 rejoined the SPLA/M as a senior commander.

Machar was re-appointed as First Vice-president of South Sudan on 22 February 2020 as part of the Revitalized Transitional Government of National Unity.

[11][12] Machar has been called a tuut dhoali/Doth in English, which may be translated "adult boy", meaning uninitiated and literate.

[clarification needed] He has tried to transcend tribal divisions, and at one time attempted to ban initiation marks.

[18] In August 1991 Riek Machar, Lam Akol and Gordon Kong announced that John Garang had been ejected from the SPLM.

[19] Kerubino Kuanyin and Faustino Atem Gualdit, Dinkas from Bahr el-Ghazal, had been among the founders of the SPLM but had fallen out with John Garang and had been jailed.

[20] In September 1993, President Daniel Arap Moi of Kenya held separate talks with Garang and Riek Machar.

The two seemed to agree about various subjects related to a cease fire and reconciliation between the two factions, self-determination and opposition to the Khartoum regime, but Machar disputed Garang's authority and refused to sign a joint declaration.

[25] He was also made commander in chief of the South Sudan Defense Force (SSDF), which included most of the ex-rebels who had signed the Khartoum agreement.

[28] In 1998–1999, Matip's fighters and government troops clashed several times with Machar's SSDF forces in a struggle for control of the Unity state oilfields.

Matip's fighters forced Tito Biel, a high-ranking SSDF commander, to evacuate Leer early in 1999.

[29] Riek Machar's failure to prevent the government from forcibly displacing civilians from the oil-producing areas of Unity State turned the Nuer against his leadership.

[25] At risk in his own homeland of the Dok Nuer, Riek moved his base of operations to the eastern Jikany area.

In August Machar became Vice President of the Government of Southern Sudan and SPLM Co-chair of the Joint Executive Political Committee.

[32] On 15 July 2011 Machar represented South Sudan at the ceremony when his country's flag was raised outside United Nations headquarters in New York.

In 2012, he publicly apologized for his part in the Bor massacre as he prepared to pave way for taking the helm of SPLM.

An exception was the Franco-Polish arms dealer Pierre Dadak who at the time of his arrest at Ibiza villa on 14 July 2016 was negotiating to sell Machar 40,000 AK-47 assault rifles, 30,000 PKM machine guns and 200,000 boxes of ammunition.

[44][45] Machar was appointed First Vice-president of South Sudan on 22 February 2020 as part of the Revitalised Transitional Government of National Unity.