Veteran actress MacLaine will appear in the hit period drama's third series as the mother of Lady Grantham (Elizabeth McGovern).
"Shirley was wonderful," Wilton told Vulture. "She's so marvellous. She's in the first two episodes, and we loved her."
The 65-year-old, who plays Isobel Crawley, also revealed that MacLaine "fell in love" with her co-star Jim Carter (Carson).
"She thought he was heaven. How could you not?" laughed Wilton.
Elizabeth McGovern was recently quoted as saying that series two of Downton Abbey was less "to [her] taste" than the first.
However, the star later insisted that her comments had been taken out of context, adding: "I was in no way criticising the second season or implying that I loved or enjoyed it any less."
Downton Abbey will return to ITV in the UK in late 2012. In the US, the series airs on PBS.