Simelane was appealing an earlier judgment by the Supreme Court of Appeal that found Zuma's decision to appoint him "irrational" and "unconstitutional".
Justice Zak Yacoob, in handing down a unanimous decision by the court – subject to a qualification by Justice Ray Zondo in relation to one paragraph in the judgment – found that Zuma and Justice Minister Jeff Radebe had failed to consider evidence before them, including the findings of the Ginwala inquiry into the conduct of then-national director of public prosections (NDPP) Vusi Pikoli and the findings of a public service commission report when appointing Simelane – rendering the president's decision "irrational".
Simelane, in his capacity as justice director general was, according to the ruling, "intimately involved" in a dispute over the proper role of the then-national director of public prosecutions, Pikoli and the powers and duties of the justice minister.
Pikoli was suspended by then-president Thabo Mbeki in Sepetember 2007 after pursuing corruption charges against then-police commissioner Jackie Selebi. On October 3, Mbeki instituted a commission of inquiry, chaired by former speaker of Parliament Frene Ginwala, into Pikoli's fitness to hold office.
Simelane had presented the government's evidence, under oath, at the commission. Ginwala subsequently criticised "with some severity" Simelane's approach to giving evidence and its credibility.
Then-justice minister Enver Surty had subsequently requested the Public Service Commission investigate Simelane's conduct in the Ginwala commission. The commission, in a detailed report, then recommended disciplinary action be taken against Simelane by the justice department. But new Justice Minister Jeff Radebe rejected the recommendations of the report, and President Zuma appointed Simelane as national prosecutor two days later.
The Constitutional Court evaluated Simelane's evidence at the Ginwala inquiry – when he was director general at the justice department – and concluded "the evidence was contradictory and, on its face, indicative of Mr Simelane's dishonesty and raised serious questions about Mr Simelane's conscientiousness, integrity and credibility".
Yacoob held this was "inconsistent" with the requirements of the position – especially in the absence of "acceptable reasons" for not considering such evidence as contained in the two reports.
Simelane, who Zuma placed on paid leave after the Supreme Court of Appeals decision in December last year, was appointed National Prosecuting Authority boss in 2009 after the 2008 Ginwala inquiry severely criticised him.
The court also held the reasons Radebe had provided for advising the president to ignore the findings of the two reports was insufficient and the president's failure to consider the evidence was "irrational" and "inconsistent with the purpose of appointing, as NDPP, a fit and proper person with due regard to his conscientiousness and integrity".
Yacoob, with one eye on the separation of powers doctrine, also noted the rationality test "was the least invasive form of judicial scrutiny and not the converse".
The application to review the decision to appoint Simelane had initially been made in the North Gauteng High Court by the Democratic Alliance (DA). The high court found in favour of the justice department and Simelane but the DA won subsequent appeals.
DA leader Helen Zille said the Constitutional Court's decision was an "important victory for every South African" and that it would protect the independence and integrity of the National Prosecuting Authority and ensure ordinary South Africans were safe from "political persecution".
Justice department spokesperson Mthunzi Mhaga said in a statement, that the department was "naturally disappointed with the judgment, however, we will abide by [it] as the Constitutional Court is the final arbiter on these issues."
Mhaga said, "Once we have received a copy of the written judgment, we will reflect on all issues raised therein in so far as they impact on the appointment of the NDPP. We are pleased that Advocate Simelane's decisions while he was NDPP remain valid."