Beresford was the illegitimate son of the 1st Marquess of Waterford. He was the brother of Admiral Sir John Beresford, 1st Baronet (who was also illegitimate), and the half-brother of the 2nd Marquess of Waterford, Archbishop Lord John Beresford and General Lord George Beresford.
Beresford entered the British Army in 1785 as an ensign in the 6th Regiment of Foot and the next year he was blinded in one eye due to an incident with a musket. He remained in the service being promoted to captain by 1791 with the 69th Regiment of Foot. He distinguished himself at Toulon (1793), in Egypt (1799–1803) and in South Africa (1805). From there crossed the South Atlantic to South America to invade the River Plate region (now Argentina), with a small British force of 1,500 men, departing on 14 April 1806. Following his move to Cape Town in Cape Colony, Beresford, spurred on by Home Popham, R.N. (later Rear Admiral Sir Home Popham), decided to attack Buenos Aires in Spanish South America. No attempt was made to gain authorization from the Crown for this undertaking. In the invasion of the River Plate, Buenos Aires was occupied for 46 days. However, the British force could not maintain itself against the army gathered by Santiago de Liniers. After a relentless two-day fight with the Buenos Aires and Montevideo militias between 10 and 12 August 1806, the British were defeated and forced to capitulate. Beresford had to surrender, remaining prisoner for six months; in the end, he managed to escape and arrived in England in 1807.
In that same year Beresford was sent to Madeira, which he occupied in name of the King of Portugal, remaining there for six months as Governor and Commander in Chief. The exiled Portuguese Government in Rio de Janeiro, Brazil, whereto the Portuguese Royal Family had transferred the Court, realised the necessity of appointing a commander-in-chief capable of training, equipping and disciplining the demoralised Portuguese Army. The Portuguese government asked Britain to appoint Arthur Wellesley to this role, Wellesley indicated he could not do the role justice due to his prior engagements and recommended Beresford. He was appointed Marshal and Commander in Chief of the Army by Decree of 7 March 1809 and took the command on 15th of the same month. At that time, French general Marshal Soult had already crossed into Portugal where he occupied Porto. Beresford quickly overhauled the Portuguese forces, bringing them in line with British discipline and organization, and from the General Headquarters (then at the Largo do Calhariz), he dispatched many "daily orders" altering points of the infantry ordnance, creating a general command of artillery, establishing the separation of the battalions, firing incompetent or corrupt officers and promoting or appointing appropriate replacements.
On 22 April 1809 Sir Arthur Wellesley, later Duke of Wellington, disembarked in Lisbon, and took over the command of all the Anglo-Portuguese troops whereupon Beresford was nominated Marshal General of the Portuguese Army. The allied armies marched to the North. Wellington moved from Coimbra directly to Porto, which he entered on 12 May, and Beresford marched through the Province of Beira, arriving that same day at the banks of the Douro river, in the area of Lamego. Wellington's troops made a forced crossing of the Douro and defeated the French, forcing their Marshall-general Jean-de-Dieu Soult to withdraw from Porto. Soult was outnumbered and expelled from Portugal; the positioning of Beresford's forces compelled the French to leave Portugal by the poor roads through Montalegre. They managed to cross the border only after sacrificing their artillery and baggage, and faced numerous difficulties during the evacuation.
The Second French Invasion of Portugal was defeated and the allied armies moved back to the South, the British concentrating at Abrantes and the Portuguese at Castelo Branco. With the intention of cooperating with the Spanish against Marshal Victor, the Anglo-Portuguese forces under Wellesley moved into Spain in the Talavera campaign while Beresford remained on the Águeda River covering the Spanish-Portuguese border. After Wellesley's return, now as Viscount Wellington, following the Battle of Talavera, Beresford re-entered Portugal, where he distributed the army at various locations and established his General Headquarters in Lisbon. From Lisbon he dispatched numerous orders and instructions for the reform of the Portuguese military.