The 25-year-old gained eligibility for the United States national team back in February, making his debut against Samoa in the 2015 Pacific Nations Cup in July, and Lam admits he has been on the GUINNESS PRO12 side's radar ever since.
He started three of the USA's four Pool B World Cup fixtures and has now booted 90 points in eight international appearances.
And although Connacht currently lie fourth in the GUINNESS PRO12 table following their 34-15 win over Zebre on Friday, Lam believes MacGinty can add something different to his troops.
"As a young Irish player playing abroad, AJ has been on our radar for quite some time now and we have been watching his games closely since he first played in the PNC during the summer," said Lam.
"His performances in the Rugby World Cup were impressive and we're very pleased that he's now joining Connacht Rugby.
"AJ adds to our options at out-half and we expect him to bring with him his experience of playing at the highest level.
"His skill-set compliments our game and he has shown that he can execute the basics of kicking and passing in high pressure situations.
"I had the chance to meet him properly last week and discuss his future. He's an extremely ambitious and determined young man when it comes to his continued development as a rugby player.
"That lined up with our own programme and what we are doing here at Connacht Rugby so we are really excited to have him on board and we know that he will add a great deal to our environment."
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