In spite of a sales uptick, the firm on Tuesday said net earnings fell 15% to $763m (Â£595m) from $898m in the same period last year.
The firm also lowered its profit guidelines for the year.
The stock price fell on the news, dipping about 3% in early trading on Tuesday.
Lockheed has come under pressure for its F-35 fighter jet program, which has been plagued by delays and cost overruns.
A report from the US government's auditing arm, the Government Accountability Office, on Monday found that "cascading delays" meant the cost of the programme would run more than $1bn over budget.
US President Donald Trump has also targeted the programme, tweeting in December that it was "out of control." He claimed that negotiations would lead to billions in savings.
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But chief financial officer Bruce Tanner said on a call with analysts that nothing significant had changed to the programme in the last six months, though talk about F-35 costs was "bundled" with pricing for some of the firm's other contracts.
Chief executive Marillyn Hewson said Lockheed profits took a hit this quarter on a defence programme for an international customer.
Lockheed also reduced the value of a joint venture in the Middle East, which is not seeing as much demand as expected.
But she said a "volatile" international environment suggests that military spending may increase, including in the US.
Lockheed sales in the first quarter were up 6.6%, to $11.1bn, from the same period in 2016.
"We remain hopeful that progress will continue to be made in this area and upcoming legislation will include the increases necessary to equip and maintain our fighting forces," she said.
Unexpected losses related to international business dragged down first-quarter profits at US defence giant Lockheed Martin.