Harow faced fraud and breach of trust charges, but made a plea bargain to testify against his former boss.
For years, as it has appeared sources in law enforcement were leaking stories against him in the media, Harow mostly held his fire.
The American-born Harow testified in relation to Case 2000, which alleges that Netanyahu sought to weaken a competitor of a paper in return for positive coverage.
Case 1000, a corruption trial in which Netanyahu is charged with fraud and breach of trust, resumed on Monday.
The Movement for Quality Government NGO petitioned the High Court, calling to limit Netanyahu's Prime Ministerial powers.