De Forest was an enthusiast for the emerging technologies of motor cars and aeroplanes. An accomplished motor racing driver, he competed in a number of major races including the Gordon Bennett Cup in auto racing.[19] From 1903–1905 he held the Daily Mail Challenge Cup, having attained a record speed over the flying kilometre at Phoenix Park, Dublin, breaking the world land speed record.
In 1909 he offered the Baron de Forest Prize of £2,000 to the first Englishman who could fly across the English Channel in an English-built aeroplane. When a Frenchman, Louis Blériot successfully crossed the Channel in July 1909, he doubled the prize to £4,000. It was eventually won by Thomas Sopwith in December 1910.
He was also a rider of the Cresta Run in St. Moritz where a cup was named after him.

De Forest was active in the Liberal Party, and at the January 1910 general election stood as parliamentary candidate at Southport. Despite the support of Churchill, De Forest was defeated by his Conservative opponent, Major Godfrey Dalrymple-White in a campaign marred by racist slurs.
In March 1910 he was elected to the London County Council as a member of the Liberal-backed Progressive Party, representing Kennington. He held the seat until 1913.
In July 1911 a parliamentary by-election was called for the seat of West Ham North, and de Forest was chosen to defend the seat for the Liberals. In his election address he stated that he was in favour of land nationalisation, Irish Home Rule, revised licensing laws, female suffrage and equality of religion in education. He retained the seat for the Liberal Party, with an increased majority

With the outbreak of war with Germany and Austro-Hungary in 1914, attempts were made to prosecute de Forest as an enemy sympathiser. However, with Churchill's assistance, he was able to resist the pressure. He joined the Royal Navy Volunteer Reserve in 1914, subsequently serving in the Royal Naval Air Service Armoured Car Section