On a muggy Sunday morning in June, I joined a few dozen people at the Adam Clayton Powell Jr. statue in the heart of Harlem. We were there to take a walk through history.

But much of that history has been erased and, as our tour guide pointed out, there were few plaques highlighting Muslim milestones in a neighborhood that has helped shape America.

The guide, Katherine Merriman, a doctoral candidate in Islamic Studies at the University of North Carolina at Chapel Hill, started highlighting the area’s past for her friends, and then for friends of those friends.

Interest kept growing and, five years ago, she opened the Muslim History Tour NYC to the public. You can find her walking backward and pointing to Muslim-related sites past and present on about a half-dozen Sundays of the year. (Demand for the free tour far outstrips supply; it took me three tries to have my R.S.V.P. accepted.)

Ms. Merriman told us about the first Muslims in New York, who arrived as slaves in the 1600s. “Muslim history is New York City history,” she said.

Most of us were stumped when she asked how many mosques were in the city today. I don’t recall anyone coming within the ballpark of 300, the correct answer.

But most of the earliest mosques are gone, and much of the history erased, so Ms. Merriman frequently turned to a wide-screen iPad to show us what had once been.