He found employment at the Belgian Finance Ministry in Léopoldville, Bukavu and worked there till the early 1950s. In 1951 he visited Angola where he witnessed Portuguese officials abusing an old man and became aware of the brutality in colonial rule.
This experience inspired him to enter politics and fight for the independence of Angola from Portuguese rule. Along with a friend Barros Necaca he founded the Union of Peoples of Northern Angola (UPNA) in 1956. It was later renamed as the Union of Peoples of Angola (UPA).
He served as the UPA president and secretly attended the All-African Peoples Congress of Ghana in December 1958 where he represented Angola. There he became acquainted with Patrice Lumumba, the future Prime Minister of the Democratic Republic of the Congo, Kenneth Kaunda, the future President of Zambia, and Kenyan nationalist Tom Mboya.
Roberto acquired a Guinean passport and went to the United Nations on behalf of his movement. In 1961 he appointed Jonas Savimbi as Secretary-General of the UPA.
In the 1950s he began receiving aid from the United States National Security Council which was initially $6,000 and later increased to $10,000.
Leading a group of 4,000 to 5,000 militants, he launched an incursion into Angola on March 15, 1961, taking over farms, government outposts, and trading centers, killing whoever they encountered enroute. More than 1,000 whites and several natives were killed in this bloody massacre.
In 1962, he merged the UPA with the Democratic Party of Angola to form the National Liberation Front of Angola (FNLA). After a few weeks he established the Revolutionary Government of Angola in Exile (GRAE) and appointed Savimbi to the position of Foreign Minister.
However some differences crept in between Savimbi and Roberto, and Savimbi left the FNLA in 1964 to found the National Union for the Total Independence of Angola (UNITA), thus making the former allies bitter political rivals.
By the mid 1960s there were three political groups fighting for freedom against the Portuguese government: FNLA, UNITA, and the Popular Movement for the Liberation of Angola (MPLA) led by writer and statesman Viriato da Cruz. The struggles continued till the mid 1970s and finally in April 1975, Roberto and the leaders of two other political parties signed peace accords with Portugal that led to Angola’s independence the same year.
Under the terms of the agreement, UNITA, the MPLA, and the FNLA agreed to form a coalition government. However, disagreements between the groups led to outbreaks of violence which ultimately led to a civil war.
The war continued for years at a stretch, going on for almost three decades in which about 500,000 people were killed and thousands others displaced. The circumstances forced Roberto to go into exile though he eventually returned to Angola in 1992.
The first multiparty elections were held in 1992 and several FNLA members took part in it; however the party received only two percent of the popular vote. The general elections were won by the MPLA and Roberto became one of the most vocal critics of the MPLA.