U.S. equities kicked off the week trading higher on Monday, unfazed by a key vote in Italy which led to Prime Minister Matteo Renzi's resignation.

The Dow Jones industrial average gained about 90 points shortly after the open to hit a new record high, with Goldman Sachs contributing the most gains. The S&P 500 advanced 0.56 percent, with financials rising more than 1 percent to lead advancers. The Nasdaq composite outperformed, trading 0.7 percent higher.

Renzi said he would step down after being defeated in a referendum regarding his plan to overhaul the Italian constitution. Renzi said voters had shown a "clear" rejection of legislative reform measures and that he would meet with his cabinet on Monday and then hand in his resignation to the President Sergio Mattarella, taking full responsibility for the defeat.

The reforms would have made it so that the executive branch needs approval only from parliament's lower house in order to pass laws. Legislation is a slow, arduous process in Italy, where there are a high number of lawmakers relative to the population, and where the two houses of parliament essentially have the same, duplicate powers.

But "going into the vote, ... the market had expected any trouble in terms of liquidity, particularly with the banks, would be mitigated by a Mario Draghi put," said Quincy Krosby, market strategist at Prudential Financial. Shares of Italian bank Banca Popolare di Milano fell more than 7 percent in European trade.

The euro briefly dipped more than 1 percent against the dollar on Sunday evening, before trading higher against the U.S. currency. As of 9:51 a.m. ET, the European common currency traded 0.6 percent higher, at $1.0731. European stocks also rose, with the pan-European Stoxx 600index rising about 0.5 percent.

"Much of the bounce back is mainly due to the victory of the Green party over in Austria which has faded the picture that there would be any outcome such as Brexit. But the pain for the euro may continue as the upcoming elections over in Germany, Netherland, France and Italy will maintain their pressure," said Naeem Aslam, chief market analyst at Think Markets, in a note. "The support of 1.02 could be tested in the coming months and we will be watching this level very closely."

Investors also payed attention to remarks made by New York Federal Reserve President William Dudley, in which he said he favors gradual rate hikes if the U.S. economy stays on track. The expectation among investors is for the Federal Reserve to hike interest rates at its meeting later in December. According to the CME Group's FedWatch tool, market expectations for a rate hike this month are above 90 percent.

In economic news, the November IHS Markit non-manufacturing index came in at 54.6, marking the ninth consecutive month in which the services sector has shown expansion. "Survey respondents noted that improved client confidence and a favourable domestic economic backdrop had helped to boost business activity in November," IHS said.

The November ISM services index read is set for release at 10 a.m. ET.

Market watchers also eyed news from the incoming presidential administration, after President-elect Donald Trump threatened payback for U.S. firms that moved abroad.

"The hope and belief of many voters is that when deciding on a candidate, campaigning is different than governing and things said doing the former are implemented more practically under the latter or not at all," Peter Boockvar, chief market analyst at The Lindsey Group, in a note to clients. "Sticking only to the economic talk (and not the SNL comments), we'll get a better regulatory and corporate/individual tax environment I believe but the trade talk was always the fear that apparently won't go away after what I read yesterday."

"I so dislike politics and I'm going to no longer discuss it after today but I have to respectively disagree with anyone that tries to convince me that this sort of trade talk is somehow a good thing," he said.

U.S. Treasurys fell across the boar, with the two-year note yield near 1.14 percent and the benchmark 10-year note yield around 2.43 percent.

In oil markets, U.S. crude for January delivery rose 0.93 percent to $52.16 per barrel amid optimism surrounding an OPEC deal reached last week.