In 1817, O’Leary travels to London to enlist in a regiment being formed by Henry Wilson. Wilson is recruiting officers and NCOs to go to South America and form a Hussar regiment in service to Simón Bolívar, who goes on to liberate much of South America from Spanish rule.
O’Leary sails for Venezuela with Wilson near the end of 1817, arriving in March 1818. Unlike many of the Irish who fight for Simón Bolívar in his many campaigns to win South American independence, he has not served in the Napoleonic Wars. He first meets Bolívar away from the front shortly his arrival and Bolívar is apparently impressed with the young Irish officer.
In March 1819, O’Leary sees his first action and is promoted to captain. In July, after Bolívar’s famous crossing through the Casanare Swamps and over the Andes, he receives a saber wound in the battle of Pantano de Vargas but he quickly recovers and takes part in the Battle of Boyacá on August 9. Shortly after this, he becomes aide-de-camp to Bolívar. Two years later, after much more fighting, Venezuela is freed.
During the next few years, as the fight continues to free the rest of South American from Spanish domination, O’Leary performs many dangerous missions for “The Liberator,” rising ever higher in his esteem. He continues to serve Bolívar well through the political and military intrigues that follow the freeing of South America from the Spanish.
After Bolívar’s death in December 1830, O’Leary disobeys orders to burn the general’s personal documents and is exiled to Jamaica by the new Venezuelan government. There he writes extensive memoirs that are later edited by his son, Simon Bolivar O’Leary, and published in the 1870s and 1880s. Simon is the eldest of six children O’Leary has with his South American wife. He also writes his own very extensive memoirs, spanning thirty-four volumes, of his time fighting in the revolutionary wars with Bolívar.
In 1833, O’Leary is able to return to Venezuela. He holds a number of diplomatic posts for the Venezuelan government for the next 20 years, and on at least two occasions is able to visit his boyhood home of Cork.
Daniel O’Leary dies in Bogotá on February 24, 1854. The Venezuelans name a plaza after him in Caracas. In 1882, they obtain permission to take O’Leary’s body from Bogotá to Caracas, where it is laid to rest in the National Pantheon of Venezuela to lie forever in death next to the man he had served so faithfully in life, Don Simón Bolívar. A bust and plaque honouring O’Leary are presented by the Venezuelan Government to the people of Cork and unveiled on May 12, 2010 by the Venezuelan Ambassador to Ireland, Dr. Samuel Moncada.