Lockheed Martin (LMT -0.45%) said Monday that Marillyn Hewson, who as CEO helped revamp the aerospace company's portfolio and turn it into one of the best-performing defense stocks, would be stepping down effective June 15.

Hewson has worked at Lockheed Martin since 1983 and was named CEO in 2013. During her tenure in charge, the company sold off much of its IT business and acquired Sikorsky helicopters from United Technologies, helping to diversify Lockheed's portfolio and making it less reliant on the F-35 Joint Strike Fighter.

A F-35 flying out over water performing a missile firing test.

An F-35 flies inverted. Image source: Lockheed Martin.

She will be replaced by board member James D. Taiclet, who has been CEO of American Tower (AMT -1.16%) since 2004. Taiclet brings aerospace experience to the role, having served as an executive at Honeywell aerospace and at Pratt & Whitney, United Technologies' engine arm. He is also a retired U.S. Air Force officer and a graduate of the Air Force Academy.

It had been expected that Hewson, 66, would be retiring in the near future. Lockheed in 2018 changed its mandatory retirement age rules to allow her to stay on past 2019. But the timing of this announcement, in a period of market turmoil, seems likely to rattle investors further.

Hewson in a statement said the time is right for her to move on, saying "the corporation is strong, as evidenced by our outstanding financial results last year and a record backlog of business."

Hewson is one of a number of high-profile executives to depart in recent weeks, joining Disney's Bob Iger and United Parcel Service's David Abney.