Not Seneca, as far as the record shows. The sentence is not in his surviving works. The earliest version on record is unsigned, in the “Fact and Comment” column of the American magazine The Youth’s Companion on 25 April 1912: “He is lucky who realizes that ‘luck’ is the point where preparation meets opportunity.” The word historian Barry Popik reports the modern wording in an Ohio newspaper in 1928, and Popik and Wikiquote both put the first credit to Seneca in 1999.
What it means
Luck is less random than it looks. Chances come along for many people, but only someone who has prepared can use one when it arrives, so what gets called good luck is mostly readiness meeting the right moment.
Where it comes from
No sentence like it is in Seneca’s Latin. The nearest passage, suggested on Wikiquote, is in On Benefits (Book VII, chapter 1), where Seneca quotes his friend Demetrius the Cynic: the best wrestler is not the one who has learned every hold, but the one who has trained well in one or two and “watches keenly for an opportunity of practising them” (Aubrey Stewart’s 1887 translation). The point is that a few precepts you can actually use beat many you cannot. Luck is never mentioned.
The trail in print is American and anonymous. The earliest version on record is an unsigned paragraph in the “Fact and Comment” column of The Youth’s Companion for 25 April 1912 (vol. 86, no. 17, p. 222), which we have seen in the Internet Archive’s scan of the issue: “He is lucky who realizes that ‘luck’ is the point where preparation meets opportunity.” Barry Popik, a word historian who collects dated citations, reports the next steps: Bankers Magazine in 1926 defining luck as “that thing which happens when preparation meets opportunity”, and the exact modern sentence in the Van Wert Daily Bulletin, an Ohio paper, on 16 July 1928. We have not seen those two pages ourselves. Popik also reports a 1991 book of daily readings for young people that credits Oprah Winfrey with a similar line, “Luck is a matter of preparation meeting opportunity,” and Wikiquote notes books naming the football coach Darrell K. Royal or the insurance writer Elmer G. Letterman. Popik and Wikiquote both put the first credit to Seneca in 1999, in a business book, and Popik notes that it names no work of his.