After his grandfather's death in 1802, the viscountcy had been inherited by his father's elder brother, Henry Monck, who in 1822 was further created Earl of Rathdowne.

However, in 1848, Lord Rathdowne died without male heirs (albeit with many daughters), the earldom became extinct, and Monck's father succeeded as 3rd Viscount.

The American Civil War had just broken out, and the Trent Affair caused diplomatic tension between the United States and Britain.

Monck supported the idea, and worked closely with John A. Macdonald, George Brown, George-Étienne Cartier, and Étienne-Paschal Taché, who formed the "Great Coalition" in 1864.

On 23 June 1844, he married Lady Elizabeth Louise Mary Monck, his first cousin and the daughter of his uncle Henry, the 2nd Viscount, who had been made Earl of Rathdowne in 1822.