In 1643 Marshall was chosen as one of the Westminster Assembly of divines, and was an extremely active and valuable member. He regularly united with his brethren in the observance of public fasts, and sometimes these meetings went on much further than any meeting we would have in our own day. On one occasion, it is said,
“that Dr. Twisse having commenced the public service with a short prayer, Mr. Marshall prayed in a wonderful, pathetic, and prudent manner for two hours. Mr. Arrowsmith then preached an hour, then they sung a psalm; after which Mr. Vines prayed nearly two hours, Mr. Palmer preached an hour, and Mr. Seaman prayed nearly two hours. Mr. Henderson then spoke of the evils of the time, and how they were to be remedied, and Dr. Twisse closed the service with a short prayer.” (Life of Marshall, p. 11.)