Despite the scorching hot weather, the comedy film starring Meryl Streep, Pierce Brosnan and Colin Firth averaged 5.66m (27.8%) for ITV1 between 7pm and 9.10pm, with a further 350k on +1.
Mamma Mia! was easily Saturday's most-watched broadcast, and ITV's most-watched movie this year, ahead of Quantum of Solace which pulled in 4.7m three weeks ago.
In stark contrast,So You Think You Can Dance's slide continued on BBC One, as the latest edition managed 3.43m (20.1%) at 6.30pm, followed by The National Lottery: Secret Fortune and Casualty which grabbed 4.28m (21.5%) and 4.73m (21.5%) respectively.
When Piers Met Andrew Lloyd-Webber won the 9pm hour with 3.64m (17.1%) for ITV1 (+1: 164k), putting it in front of BBC One's Live at the Apollo repeat, which had 2.86m (13.4%).
Elsewhere, BBC Two's four-hour Masters golf coverage scored 1.46m (8.6%) between 8.30pm and 12.30am, while Transporter 3 notched up 1.88m (9.3%) between 9.15pm and 11.15pm, and 344k for Channel 4 +1.
Predictably, CSI: NY was Channel 5's best-performing programme with 1.76m (8.2%) at 9.05pm, while Legally Blonde led the multichannel ratings with 787k (3.9%) for BBC Three at the same time.
Overall, ITV1 led BBC One in primetime with 21.1% (+1: 1.2%) to 19.3%. Channel 4 was in third place with 6.9% (+1: 1%), followed by BBC Two with 6.3%, and Channel 5 with 5.3%.
BARB ratings data supplied by Attentional