In 2014, Old Dominion University and George Mason University published a study on prevalence of noncitizen voting
The researchers estimated that 6.4% of noncitizens voted in the 2008 presidential election and that 2.2% voted in the 2010 midterm election. Of the 2008 estimate, the authors wrote, "Since the adult non-citizen population of the United States was roughly 19.4 million (CPS 2011), the number of non-citizen voters (including both uncertainty based on normally distributed sampling error, and the various combinations of verified and reported voting) could range from just over 38,000 at the very minimum to nearly 2.8 million at the maximum."
In their conclusion, the authors wrote the following:
“These results speak to both sides of the debate concerning non-citizen enfranchisement. They support the claims made by some anti-immigration organizations that non-citizens participate in U.S. elections. In addition, the analysis suggests that non-citizens’ votes have changed significant election outcomes including the assignment of North Carolina’s 2008 electoral votes, and the pivotal Minnesota Senate victory of Democrat Al Franken in 2008.
However, our results also support the arguments made by voting and immigrant rights organizations that the portion of non-citizen immigrants who participate in U.S. elections is quite small. Indeed, given the extraordinary efforts made by the Obama and McCain campaigns to mobilize voters in 2008, the relatively small portion of non-citizens who voted in 2008 likely exceeded the portion of non-citizens voting in other recent U.S. elections."
https://ww2.odu.edu/~jrichman/NonCitizenVote.pdf