He was a founding member of the Society of Ethical and Religious Vegetarians (SERV),[4] and a former outreach director of the Fund for Animals.

He could love, he could trust, he could share, he could enjoy, he could fear, he could worry, he could look forward to the future and remember the past, he had a sense of who he was, and he would have sacrificed himself for me without a moment's hesitation.

In 1994, Phelps retired from the federal government and joined the campaigns office of The Fund for Animals in Silver Spring, Maryland, where he became active in the campaign to end the live pigeon shoot which was then held every Labor Day in the village of Hegins, Pennsylvania.

From 2002 until his death in 2014, Phelps had suffered from myasthenia gravis, an auto-immune neuromuscular condition that causes severe fatigable weakness.

Although he is generally opposed to militant direct action on the grounds that it is counterproductive, Phelps supported live rescues of animals from farms and laboratories.