On Friday night, Mr Mercer – who is a former veterans minister – said he had boarded a plane to Krakow “a week ago” and headed back to the West on Wednesday.
The Plymouth Moor View MP tweeted photographs of himself visiting injured people in hospital, adding he had witnessed “total carnage” alongside demonstrations of “incredible human spirit”.
Mr Mercer told The Times: “I didn’t tell anyone, I just disappeared. I decided it was the right thing to do.”
Writing in the Daily Telegraph, he said he made the decision to travel after receiving an invitation from a former MP in the Donetsk region.
Government advice warns against travelling to the country. Mr Mercer is the first UK MP to visit the country’s capital since the outbreak of war.
Conservative MP Adam Holloway was criticised by Downing Street last week for visiting a different part of Ukraine.
Mr Mercer said he also visited Lviv, where he said he met with local politicians.