When asked about his job at cocktail parties, Alan Johnson has a curiosity-piquing line.

“You know those big paydays on Wall Street?” he says, typically waiting a beat to deliver the punch line. “I have something to do with them.”

Mr. Johnson, a consultant who speaks with a light twang from his native Alabama, has never worked for a bank. Nor will his company, Johnson Associates, pay million-dollar bonuses to any of its 12 employees this year. But as one of the nation’s foremost financial compensation specialists, Mr. Johnson is among a small group of behind-the-scenes information brokers who help determine how Wall Street firms distribute billions of dollars to their workers.

“The misunderstanding many people have about this industry is that pay is whimsical,” Mr. Johnson said in a recent interview at his company’s Manhattan office. “It’s not.”

Compensation consulting is an obscure corner of the management consulting industry, where practitioners operate in the shadows of high finance. Large Wall Street banks, as well as hedge funds and private equity shops, rely on such consultants to help them structure bonus payouts and devise severance packages, and to provide data on what competitors pay.