In order to understand NCIS's slow start, it's important to know the background. In 1995, NBC picked up a show named JAG (Judge Advocate General), a drama series about the US Navy. It premiered Saturday at 8pm, which is basically what Friday is today. It had a solid lead-out, The John Larroquette Show, which came in #42 in the season, yet incompatible. Asking a new show to lead off a Saturday is always hard business, and once NBC realized it had the inability to do so, it was moved to Wednesday at 8pm, a night which also included Dateline at 9pm and Law & Order at 10pm, both
top 30 shows. JAG was basically to NBC's Wednesday what The Mysteries of Laura is today. JAG averaged 11.56 million viewers, but only came in as #79 for the season. As such, NBC cancelled it and moved on with their lives.
But not so fast--CBS saw some potential in JAG, and picked it up to anchor Friday nights as a midseason replacement, airing after new sitcom entry Everybody Loves Raymond. The struggling Raymond didn't make it long on Friday nights, upgraded to the post-Cosby spot on Mondays, meaning that JAG took over Fridays at 8 soon thereafter. It still struggled in its second season, barely improving on its first, with an average of 11.80 million viewers and ranking #68 for the season.
Apparently CBS still saw potential in the show, giving it a better time slot (Tuesday at 8) for a third season that presumably many did not see coming. Naturally, it saw significant gains, rising to 12.90 million viewers and ranking #36 for the season. In season 4, it moved up in the rankings to #17 and 14.20 million viewers, marking the first time it cracked the top 20. For the next three seasons, rankings went up and down (#26, #15, #26) while viewership remained between 14 and 15 million for the most part. This is all despite staying in its same Tuesday time slot with no lead-in, meaning that CBS must have made a big promotional push, and word-of-mouth must have spread.
In fact, the show that ranked #79 for its first season went into season 9 with its first-ever spin-off: NCIS, which would take over its Tuesday at 8 time slot, sending JAG back to Fridays for its final two seasons.
NCIS
Unfortunately, the JAG spinoff flopped. Averaging just a 2.73, the outlook did not look bright for NCIS. According to SpottedRatings, a 2.73 in the 2003-2004 season would be equivalent to a 1.13 today. Even at its high, NCIS was only at 76% of the scripted average, and at the low, 52%. That low is even worse than Undateable relative to NBC's average today.
However, due to the network struggling overall, presumed overseas sales, and the fact that something had to face American Idol, NCIS was renewed for a 2nd season. CBS must have been at least a bit pleased at its Season 2 results--although the show was still marginal and never broke the network's average, it also rose 20% year-to-year. And another 20% in Season 3, to an average of 3.92, or about a 1.62 today. For the next two seasons after that, it remained a reliable performer, despite dropping a bit in the ratings. And all this without any lead-in support, getting the same Tuesday at 8 time slot that made it a flop in its first season.
After having an impressive Season 6, where every single rating was above average for the network, it really broke out in its seventh season, where it rose 12% in raw ratings and hit a series high of a 4.8.
That season remains its best yet in raw ratings, averaging a 4.07 in the A18-49 Live+Same Day demo. This may in part have been due to getting its very first spin-off, NCIS:LA, which aired directly after the parent show. Who would have thought that a show that flopped out of the gate, which was the spin-off of another struggling show, would get its own spin-off?
In the 2010-2011 season, it only lost 2% in raw ratings, and at that point it was really considered to be one of the biggest hits on television. Today, it despite double digit ratings declines in each of the last three seasons, the 13th(!) season of NCIS remains one of CBS's top shows. As of this publishing (January 6, 2016), NCIS is the #9 show on all of broadcast, and the #2 show on CBS, only behind The Big Bang Theory. NCIS:LA has declined hard in its move to Monday, but a second NCIS spin-off has been launched (NCIS:NO) that has done fairly well in the ratings, and just tied a series high in a crossover with the parent show. NCIS's rise is definitely one of the most remarkable in recent drama history, and I can see this show finishing with at least 15 or so seasons.