According to Thisdayinmusic.com:
July 13th, 1974, George McCrae started a two week run at No.1 on the US singles chart with 'Rock Your Baby', his only US No.1. Regarded by some as the first Disco No.1.
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I'm not sure if I'd agree, I've generally considered Hues Corporation's 'Rock The Boat' to be pretty disco. That was #1 the week before "Rock Your Baby". But you could make an argument for TSOP by MFSB, which was #1 in April of 1974. Or Perhaps even's "Love's Theme" by Love Unlimited Orchestra (Barry White) which was #1 in February 1974.
What do you think?