Adekunle was born in Kaduna. His father was a native of Ogbomosho, while his mother was a member of the Bachama tribe. He underwent secondary education at the government college, Okene (in present day Kogi State). He enlisted in the Nigerian Army in 1958 shortly after completing his school certificate examinations. He passed the army selection examinations and was accepted at the prestigious Sandhurst military academy, where he and his course mates were imbued with a thorough academic grounding in the art of warfare. He was commissioned 2nd Lieutenant on December 15, 1960. As a platoon commander, he served in the Kasai province of Congo with the 1QONR (1st Battalion) during his first tour of duty with UN peace keeping operations in that country (ONUC). In 1962, Lt. Adekunle became Aide-de-Camp to the governor of the eastern region, Sir Akanu Ibiam. The following year, as a Captain, he was posted back to the Congo as Staff Captain (A) to the Nigerian Brigade HQ at Luluabourg - under Brigadier B. Ogundipe. In 1964, Major Adekunle attended the Defence Services Staff College at Wellington, in India. When he returned he was briefly appointed Adjutant General at the Army Headquarters in May 1965 to replace Lt. Col. Yakubu Gowon, who was proceeding on a course outside the country. However, he later ceded the position to Lt. Col. James Pam and was posted back to his old Battalion (1st Bn) in Enugu as a Company Commander.
He later assumed command of the Lagos Garrison as a substantive Lt. Col. When the Biafra civil war erupted in July 1967, Adekunle was tasked to lead elements which included two new battalions (7th and 8th) - to conduct the historic sea borne assault on Bonny in the bight of Benin on 26 July 1967 (carried out by Major Isaac Adaka Boro's unit). This happened after the federal government gained confidence of most south western ethnic groups as a direct result of Biafran push to mid-west state and probe into Western region. Adekunle was promoted to Colonel after the Bonny landing.
The 6th (under Major Jalo) and 8th (under Major Ochefu) battalions of the Lagos Garrison subsequently took part in operations to liberate the Midwest following the Biafran invasion of August 1967. The 7th (under Major Abubakar) stayed behind to hold Bonny. Because Major Jalo's outfit was seconded to Lt. Col. Murtala Mohammed's 2nd Division, Adekunle was left with only the 8th Battalion at Escravos. He, therefore, protested to Army HQ and got the Lagos garrison upgraded to Brigade status through the creation of the 31 and 32 Battalions (under Majors Aliyu and Hamman, respectively). This formation, combined with elements of the Lagos garrison along the eastern seaboard, was officially designated the 3 Infantry Division. However, Colonel Adekunle did not think the name "3 Infantry Division" was sensational enough nor did it project the nature of the unique terrain in which his men had to fight. Therefore, without formal approval from Army HQ, he renamed it the " 3 Marine Commando (3MCDO)." The "Black Scorpion" as he came to be known, was easily the most controversial, celebrated and mythologized figure in the war of attrition that laid the foundations for Nigeria's contemporary crisis; and threw a wedge into the national fabric.
On opening the war front, the now 3MCO was able to push the Biafra army inward and clear it off most of the ethnic minorities area, allowing the new 12 state structure of Federal Government to be implemented. This also undermine Biafran dominant ethnic group, the Ibos in influencing the other ethnics in the area. Two governors of the two new minority state were able to be installed and functional after the 3MCO offensive. Adekunle popularity during Nigeria civil war was due to the speed at which he defeated Biafran army in major battles and speed of advances. However, the advance became stalled and Biafra was able to push back and recaptured Owerri. Also, Biafra was able to launch various ambushes while the division gain little. This was the state of 3MC before Adekunle was redeployed back to Lagos for a new commander, Colonel Olusegun Obasanjo who latter use the troop to defeat Biafra. What was remarkable was that, the division came late into the war and gained more territory compare to other divisions fighting from the north of Biafra.
Adekunle was latter put in charge of decongesting Lagos port that was having a chronic problem of clearing imported goods. This was his position before he was put on compulsory retirement.
He attributed his problem during and after the war to his rivals in army. In his various interviews, he said there always a rumour of coup linked to him until the army authority feel concern to do something about it. He had large followings in both the army and public at large and is the most popular military commander during the war apart from Obasanjo who succeeded him and brought the war to an end with same 3MC.
Adekunle led the Third Marine Commando Division with such great panache and determination, and the foreign media, in looking for human angle stuff about the Biafran war found Adekunle a ready source of news. Benjamin Adekunle was promoted Brigadier-General in 1972. On August 20th, 1974, he was compulsorily retired from the Army.
Gen. Adekunle's son wrote an account of his father's military career in a biography titled The Nigeria-Biafra Letters: A Soldier's Story.