As an English name, Monica has been in general use since the 18th century. In America it reached the height of its popularity in the 1970s, declining since then. A famous bearer was the Yugoslavian tennis player Monica Seles (1973-).
It has occasionally been used as an English given name since the 16th century. It was moderately popular in the 1940s, but had a more notable upswing in the early 2000s. This may have been inspired by the Spanish actress Penélope Cruz (1974-), who gained prominence in English-language movies at that time. It was already rapidly rising when celebrities Kourtney Kardashian and Scott Disick gave it to their baby daughter in 2012.
The name was common among Jews in the Middle Ages, but it was not generally used as a Christian name in the English-speaking world until after the Protestant Reformation. It was moderately popular in the first half of the 20th century, but starting in the 1960s it steadily rose, reaching highs in the 1980s and 90s. The character Rachel Green on the American sitcom Friends (1994-2004) may have only helped delay its downswing.
Notable bearers include American conservationist Rachel Carson (1907-1964), British actress Rachel Weisz (1970-), and Canadian actress Rachel McAdams (1978-).