In 1913 Southern Illinois Light & Power Company (SIL&P) was formed to consolidate the utility companies serving a few small communities east of St. Louis. By 1919 SIL&P had added more than a dozen communities to its service area when it was acquired by the North American Light & Power Company, which had been formed four years earlier.

In 1923 North American Light & Power acquired the Illinois Traction Company and then consolidated it with SIL&P, to form the Illinois Power & Light Corporation (IP&L). IP&L corporate offices were established in Chicago and William B. McKinley was named chairman of the board. Clement Studebaker Jr. was named president and H.L. Hanley was elected vice-president and general attorney.

At the time of IP&L's incorporation, North American Light & Power was part of two larger holding company structures, the Insull group and the North American Company, which each owned a 50 percent stake in North American Light & Power Company and represented the third and fourth largest utility holding structures in the United States at the time.

In 1934, North American Power & Light was folded into the North American Company under the NAC name.