Former Deputy Attorney General Rod Rosenstein kicked off post-government life by slamming former FBI Director James Comey, calling him a “partisan pundit” whose firing was justified.

Speaking to a group of city officials in Baltimore on Monday, his second day as a private citizen, Mr. Rosenstein defended a memo he wrote in May 2017, which the White House initially cited as grounds for ousting Mr. Comey. As the head of the Federal Bureau of Investigation, Mr. Comey was leading its probe into possible Trump campaign ties to...