(WSJ) - JUST ABOUT ANYONE who has ever opened a bottle of wine has probably wondered about, and perhaps even envied, a winemaker’s job. It’s one of those professions that sounds terribly glamorous—an entree into a privileged world. The fact is it takes time, luck and a lot of heavy lifting (literally) to ascend to the top, and it almost always involves some not-so-glamorous interim jobs.

The assistant winemaker stands on the penultimate rung of the ladder and is the unsung and invariably invisible force behind a great winemaker and winery. He or she performs much of the necessary drudgery that goes into the creation of an exceptional wine, from cleaning and topping the barrels to “racking” them (moving the wine from one barrel to another). There’s a lot of data-entry work, too.

By assuming such tasks, assistants free head winemakers to focus on the “intellectual” part of the job, including tastings to determine the final blend of a wine, said MJ Tsay, assistant winemaker at Realm Cellars, a top Cabernet estate in Napa Valley. “The higher up the ladder you are, the less physical work you do,” she added. Though, of course, that depends on the winery; at small estates, assistant and perhaps even head winemakers will do more physical labor than those who work at big wineries.

The 37-year-old Ms. Tsay was one of several assistant winemakers I met on a trip to Napa last month. A chemistry and French major in college who later earned an enology degree, Ms. Tsay has been at Realm Cellars for four years.

That’s a typical amount of time to remain in the job, according to Philippe Melka, one of Napa’s superstar consultant winemakers. “Most assistant winemakers stay three to five years,” before taking on head winemaker jobs of their own, said Mr. Melka, whose two assistants and director of winemaking at Melka Wines also help look after 15 clients at his consulting firm, Atelier Melka.

One of his assistants, Marie-Laure Ammons, is an exception to this rule. Ms. Ammons, who trained in Bordeaux and Burgundy, has been with Mr. Melka more than 11 years. She said she has stuck around because she is still learning from Mr. Melka, especially about blending wines. Blending is a particular challenge for aspiring winemakers; it requires a great deal of experience and insight to know which combination will be just right.

Ms. Ammons gets extra practice by making wine on the side for a few private clients—one of the perks that many assistants earn after working with a winemaker for a while. Using the facilities of a winery client, they can make wine without the risk or expense of a solo venture. (Grapes are expensive and vineyard prices in Napa are exorbitantly high.) They also don’t have the responsibility of selling the bottles—something assistant winemaker Andy Jones is glad he doesn’t have to worry about, at least not yet.

Mr. Jones is one of four assistants working with Thomas Rivers Brown, another superstar Napa winemaking consultant. Mr. Brown, whose client roster includes 25 wineries, also makes much-sought after Pinot Noirs and Chardonnays under his own Rivers-Marielabel.

Mr. Jones joined the team in 2008 as a harvest intern. A typical starting point for would-be assistant winemakers, harvest interns do the most basic and brutal work, which almost always involves cleaning—cleaning hoses and cleaning barrels over and over and over again. He was promoted to assistant winemaker in 2011 after three years working in the cellar, a time frame that Mr. Brown also called typical, although he noted that some aspiring vintners—particularly those who come to the industry later in life after other careers—might think it is too long to wait. Graduates of prestigious viticulture and enology programs like that of the University of California, Davis, can also be impatient, Mr. Brown added. “Some Davis grads think they should get an assistant winemaking job” right out of college, he said. “It just doesn’t work that way.”