Consumer activist Susan Bennett King, 38, has been named by President Carter to head the Consumer Product Safety Commission - thus becoming the first Democrat to direct the five-year-old agency.

In making the announcement yesterday, President Carter said he personnally endorsed a three-year reauthorization for the beleagured commission, which has been the target of criticism from consumers, industry and Congress that it is ineffective.

Carter said, too, that he supported the Office of Management and Budget request that the CPSC be given a $41.5 million budget for fiscal 1979, $1.5 million above the funding level approved by the House Appropriations Committee.

After Republican Chairman S. John Byington announced earlier this year that he would resign on June 30, Congress seemed more receptive to continuing funding for the agency.

Under Byington's controversial two-year term, the agency came under criticism for setting too few standards and for several alleged civil service violations. Byington was credited with a drastic management reorganization of the loose bureacracy he inherited.

But with the failure of the administration to get the Consumer Agency Bill through Congress this year - an action that would have created a central agency for consumer advocacy in government - it was felt that a further dismantling of the CPSC could be politically disastrous.

In a 20-minute meeting with King on Wednesday, Carter said he supported continued funding for the agency at least at the present level, plus inflation. For the past five years, each CPSC budget increase has not kept up with inflation, causing it to lose ground in terms of staffing and resources.

In a statement released yesterday, King said she was "honored by today's White House announcement . . . My appointment comes at a critical juncture in the agency's history. Whether the agency should remain in existence was recently the subject of close scrutiny, both by the White House and the Congress."

Noting that now both Congress and President have indicated support for reautorization, she added, "I am heartened by this and embrace the challenge."

She said she is starting immediately to review the programs and priorities of the agency. "Success," she said, "depends on our ability to narrow our efforts to the most significant consumer hazards and to focus our resources on sound and defensible regulatory efforts."