During his period at the Ministry of Health, Greenwood raised widows' pensions and through the Housing Act 1930 enacted large-scale slum clearance.

"[2] On 2 September 1939, acting for Attlee who was in hospital for prostate surgery, he was called to respond to Neville Chamberlain's ambivalent speech on whether Britain would aid Poland.

The moment that act of aggression took place one of the most important treaties of modern times automatically came into operation … I wonder how long we are prepared to vacillate at a time when Britain, and all that Britain stands for, and human civilisation are in peril.When the wartime coalition government was formed, Winston Churchill appointed him to the War Cabinet as Minister without Portfolio in 1940.

He was generally seen as ineffectual, but in May 1940 he emerged as Churchill's strongest and most vocal supporter in the lengthy War Cabinet debates on whether to accept or reject a peace offer from Germany.

[7] From February 1942 until the end of World War II, Greenwood also performed the function of Leader of the Opposition, though he did not receive the salary.