$Apple Inc(AAPL.US)$ posted the biggest spring-quarter profit in its 45-year history, leading a streak of record-setting earnings for technology companies even as the pandemic continued to evolve.

Not to be overlooked, $Alphabet Inc-CL A(GOOGL.US)$ and $Microsoft Corp(MSFT.US)$ also said Tuesday that their quarterly earnings excelled, underscoring how the pandemic has shifted life and work online and strengthened some of the world's biggest and wealthiest private enterprises.

Apple earned $21.7 billion in profit for the three-month period that ended in June, while revenue rose 36% from a year earlier to $81.4 billion, beating expectations.

The Cupertino, Calif., iPhone maker is on pace for its best fiscal year ever with a projected profit of $86 billion for the 12-month period ending in September, according to an average forecast of analysts surveyed by FactSet. That would be about 51% better than last year's record; analysts a year ago were predicting what now seems like a paltry 11% gain.

"This quarter saw a growing sense of optimism from consumers in the United States and around the world, driving renewed hope for a better future and for all that innovation can make possible," Apple Chief Executive Tim Cook said. "But as the last 18 months have demonstrated many times before, progress made is not progress guaranteed."

Despite the record results, Apple shares fell more than 2% during after-hour trading after the company warned that its rate of growth won't continue, an indication that investors are concerned about whether tech companies can hold on to the gains of the past year.

"Investors believe the good times will soon slow," Gene Munster, managing partner at Loup Ventures, a venture-capital firm specializing in tech research, said. "At the root of tonight's sell off is the question of the sustainability of the strength. The numbers are so impressive, comps get difficult and it's hard for investors to believe the epic growth will continue."

Investors came into 2021 expecting big things from Apple, thanks to the iPhone 12 lineup and its 5G cellular connectivity. The company has outdone even lofty expectations, benefiting from two developments: the first iPhone in three years with technological advances perceived by longtime customers as worth a new purchase, and accelerated demand for laptops and iPad tablets from workers and students stuck at home during the coronavirus pandemic.

Sales from iPhones rose 50% to $39.6 billion during the April-to-June period compared with a year earlier. "The iPhone 12 family continues to do incredibly well," Chief Financial Officer Luca Maestri said in an interview.

While iPhone sales traditionally fall off in the year after a big launch, the excitement of 5G may carry through fiscal 2022. "With close to 1 billion iPhones in use globally, the 5G upgrade opportunity remains significant, with current U.S. carrier promotions an added tailwind," William Power, an analyst for Robert W. Baird & Co., wrote in a note to investors this week.

For the year ending in September, analysts have predicted Apple's total iPhone revenue would rise 36% compared with fiscal 2020 and stay at roughly the same level in 2022 before declining a bit in 2023.

"We're not predicting the next cycle," Mr. Cook said, but then noted, "We're in the very early innings of 5G."

A top concern for investors in Mr. Cook's remarks to Wall Street analysts Tuesday is his outlook for the current quarter and details about how the company is handling the microprocessor shortage now roiling the tech and auto industries.

So far, Apple has been able to avoid big disruptions. But in April, the company cautioned that the usual drop in sales in the third quarter compared with the second quarter would be greater than normal. At the time, Mr. Maestri attributed the trend, in part, to an expected chip shortage, and warned it could hurt sales by as much as $4 billion. As it turned out, sales fell 8.5%, with Mr. Maestri putting the hit from the shortage at "just below $3 billion."

Sales in fiscal 2019 fell 7% between the second and third quarters while declining 13% in those periods in 2018. The outlier was last year when sales fueled by pandemic buying rose 2.4%.

Part of the drop between this year's quarters is because of stronger-than-usual demand for the iPhone in the quarter that ended in March. The launch of the iPhone 12 lineup with 5G cellular connectivity was delayed in 2020 because of the outbreak of Covid-19.

The current quarter, however, may be more challenging as Apple navigates the supply-chain issues. The company's revenue growth won't be as strong as it was in the April-June period in part because of supply chain issues, Mr. Maestri told analysts. Still, he added, "We are expecting to grow very strong double digits."