Johnson, who celebrated his 83rd birthday July 3, issued a formal statement late Friday, after he had privately begun informing party leaders in the last several days.
"It is time to put my family and health first," said Johnson, noting the decision came after "deep discussion" with his wife, Christel, and family, which recently gave him a second grandchild.
Senate Majority Leader Dean Skelos called Johnson "an institution in Suffolk County" who "has left an indelible mark on this chamber" as onetime head of the finance committee who helped provided middle-class tax cuts and pass a state property-tax cap.
Johnson's decision comes only days after Suffolk Legis. Ricard Montano (D-Brentwood) made a surprisingly strong showing Tuesday by filing 2,567 signatures on nominating petitions to run for the seat, without any help from the Democratic Party.
"I'm pleased for him," Montano said of Johnson. "He's had a long, distinguished career. He's done great service for the county. It's a good move for him and his family at this time."
Others say Republicans had hoped Montano would not follow through in gathering petitions. "They expected another walk in the park," said Desmond Ryan, veteran Albany lobbyist. "They never anticipated the number of signatures Montano got and are petrified about the changes in enrollment, ethnic makeup and a large presidential turnout."
Democrats outnumber Republicans in the 4th Senate District, 62,058 to 60,189. Hispanic voters make up 18.5 percent, and blacks, another 9.5 percent.
"Owen was torn because he's the consummate team player," said John Jay LaValle, Suffolk GOP chairman. "But after his birthday his family was pushing him to retire . . . In the end, he just did not want to stay."
LaValle said he will huddle with Islip GOP chairman Frank Tantone and Babylon GOP chairman Anthony Pancella to discuss a potential replacement for Johnson, and has invited Skelos to attend. The deadline to name a successor is next Friday.
LaValle said GOP contenders include Assemb. Philip Boyle (R-Bay Shore), Islip Supervisor Thomas Croci, Suffolk Legis. Thomas Cilmi (R-Bay Shore), Islip Town Board member John Cochrane Jr. and Lindenhurst village Trustee Kevin McCaffrey, a Teamster union official.
However, speculation also has surfaced over other names, including former Suffolk County Executive Steve Levy, Islip Town Board member Trish Bergin Weichbrodt and former Islip Town Board member Pamela Greene, because candidates do not have to live in district during a reapportionment year.
Suffolk Conservative chairman Edward Walsh expressed unhappiness that GOP leaders would meet without including him. "It's a little odd that in a race like this, where they may get 15 percent on our line, that Skelos and Republican leaders are running by themselves."
He said Conservative officials will meet over the weekend to decide whom to support.