The president of Merrill Lynch Wealth Management believes that Wall Street's incredible 2019 performance is nowhere near the end of the longest and best-performing bull market ever.

"The trend is your friend. We're very bullish on the market right now," Merrill Lynch's Andy Sieg told CNBC on Thursday — predicting the S&P 500 could soar another 20% from this week's record highs.

The S&P 500 opened higher Thursday and hit another all-time intraday high in late morning trading. It fell just 1 point on Wednesday from Tuesday's all-time high close. The index's more than 27% gain so this year is best since 2013.

"Our upside scenario from our research department today has the S&P 500 at 3,500 and change ... mid-next year," said Sieg. The S&P 500 at 3,500 would be 9.6% higher than Wednesday's 3,192.52 close.

"Where we see things lining up right now, we're actually more bullish then that," Sieg explained in a "Squawk Box" interview. "There's an upside scenario that could see the market up 20% from these levels."

To make his case, Sieg said he's encouraged by what's often seen as a counterintuitive signal.

"One of the things we have insight to through our 15,000 financial advisors is just what's the sentiment among Main Street investors," he said. "Investors are largely wary of markets."

Often times when individual investors, whose feelings tend to lag the market, are worried about stocks, professionals see that as a buying opportunity.

"We're probably more mid-cycle in terms of this bull market and expansion" in the economy, argued Sieg. He's also part of the executive management team at Bank of America, parent company of Merrill Lynch.

Merrill Lynch manages over $2 trillion in client assets.