The Australian international netted his first goal since August to help Boro to a vital 1-0 victory over Watford on Saturday and McMahon said it was great for the former Celtic man to get back on the scorers list.
"I'm over the moon for Scott," McMahon told the Evening Gazette.
"It's been a very frustrating time for him over the last couple of months because he has deserved a goal or two.
"He's been working hard and doing everything right for the team but just not scoring goals. I just hope they start to flow for us now."
And McMahon defended McDonald's irritable reaction to being subbed off with just under 20 minutes remaining.
"He's usually grumpy after games but he was a bit happier after this one," added McMahon. "To be fair, he's still a bit grumpy because he likes a moan on and off the pitch.
"He always wants to do better for himself and the team."